Certain amphiphilic molecules have the ability to self-assemble into microstructures, typically having an elongated, generally cylindrical geometry. Tubules have been reported to form from phosphatidylcholines bearing a diacetylenic unit in their hydrophobic chains (Schnur J M et al, U.S. Pat. No. 4,877,501) or from diacetylenic phospholipids in which the choline group is replaced by hydroxyethanol (Markowitz M A, Schnur J M and Singh A, Chem. Phys. Lipids 62, 193, 1992). In most cases chirality was believed to be a necessary feature to promote the formation of tubules (Schnur J M, Science, 262, 1669, 1993), although tubules formed from non-chiral amphiphilic ammonium salts with rigid-rod segments in the hydrophobic tails were also reported in a few cases (Kunitake T, Agnew. Chem. Intl. Ed. Engl. 31, 709, 1992; and Kunitake T., et al, J. Am. Chem. Soc., 103, 5401, 1981). Other hydrogenated species are disclosed in Singh A, Schoen P E, and Schnur, J M (1988) J. Chem. Soc. Chem. Commun. 18:1222; Fuhrhop, J H et al. (1991) J. Am. Chem. Soc. 113:7437; (1993) Chem. Rev. 93:1565; and (1993) J. Am. Chem. Soc. 115:11036; Okahata Y and Kunitake, T. (1979) J. Am. Chem. Soc. 101:5231; Nakashima N. et al, (1985) J. Am. Chem. Soc. 107:509-510; Kuwahara H. et al., (1993) J. Am. Chem. Soc. 115:3002; Ishikawa Y. et al, (1990) Chem. Lett. 25-29; and Yanagawa, T. et al, (1989) J. Am. Chem. Soc. 111:4567.
Tubules assembled from hydrogenated amphiphiles have been reported to encapsulate diverse materials (Burke T G, Singh A., Yager P. Ann. N.Y. Acad. Sci. 507, 330, 1987) and to have potential, when coated with a metal, as controlled release systems or in microelectronics (Schnur J M, Science, 262, 1669-1676, 1993).
Helixes have been reported to form from chiral zwitterionic hydrogenated amphiphiles containing amino acid units (Kunitake T. Angew. Chem. Intl. Ed. Engl. 31, 709, 1992) or from chiral aldonamides (Fuhrhop J H et al, J. Am. Chem. Soc. 109, 3387, (1987)).
Tubules are defined as hollow, cylindrical structures composed of one or several bilayers of amphiphiles. Typical diameters are in the range of from about 0.05 to 3 microns and the internal space depends on the number of bilayers. Tubule length and aspect are controllable parameters which depend on fabrication procedure (U.S. Pat. No. 4,877,501). Helixes are defined as spiral-shaped elongated structures. They are believed to be involved into tubule formation (J H Fuhrop et al, Chem. Rev. 93, 1565, 1993).
Self-assembled microstructures other than tubules and helixes include fibers, that can consist, for example, of strings of elongated aqueous compartments limited by bilayers or of bicontinuous cubic phase aggregates made of the appropriate amphiphiles.
Tubules formed from diacetylenic lipid monomers lack chemical, thermal and mechanical stability. Rapid polymerization by gamma rays can overcome this disadvantage; however, polymerization has been reported to result in a loss in chain packing and in the presence of defects (Schoen P E, Yager, P., J. Polymer Sci., Polymer Physics Ed. 23, 2203, 1985). Poor chain packing is known to increase the permeability of the membrane (Ringsdorf R. et al, Angew. Chem. Intl. Ed. Engl. 27 113, 1988) which is an inconvenience when tubules are used for encapsulation. Moreover polymerization can destroy or denature radiation sensitive materials such as proteins (Yager P., Biosensors 2, 363, 1986).